Method and apparatus for controlling grinding processes

ABSTRACT

A grinding machine is shown as including a machine base, a wheel supported by the base and carrying a rotatable superabrasive grinding wheel, a device for relatively feeding the grinding wheel and a workpiece at a feed rate along a normal force vector between the two, and an apparatus for controlling the grinding processes. The apparatus further preferably includes a force transducer mounted adjacent the wheelhead to measure the magnitude of the normal force factor occurring between the wheel and the workpiece, as well as a control device for varying the feed rate and for determining the relative wheel sharpness based upon the feed rate and the measured normal forces. The feed rate can thereby be varied in response to the determined wheel sharpness to maintain a substantially constant normal force and to optimize the grinding process. By monitoring the magnitude of the normal force factor and by determining the wheel sharpness, the grinding machine of the subject invention enables automatic conditioning of a superabrasive grinding wheel, optimization of the grinding process, and optimization of reshaping of the grinding wheel.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of copending application Ser. No. 07/400,733 filedon Aug. 29, 1989, allowed, and a continuation-in-part application of theprior copending application entitled "Method and Apparatus forControlling Grinding Processes," Ser. No. 07/240,021, filed Sept. 2,1988, now abandoned, in the names of the present applicants.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to grinding machines employing a rotaryabrasive wheel for carrying out grinding processes and, in particular,the invention relates to a machine and process for automaticallycontrolling and optimizing grinding with a superabrasive grinding wheelby monitoring the normal forces imposed on such grinding wheel.

2. Background Information

It is known in the grinding industry that superabrasive wheels, forexample, cubic boron nitride (CBN), will tend to increase in sharpnessafter trueing as successive workpieces are ground, and thereby willresult in higher metal removal rates for an equivalent normal forceapplied to the wheel, causing variations in sizing and finish.

It is well known that reconditioning (either dressing or trueing) asuperabrasive wheel causes it to behave in a dull fashion, since thegrit is leveled with the bonding material. If high force is applied to adull grinding wheel, wheel damage or part burn may occur. Through avariety of grinding actions, the bonding material becomes eroded as thewheel progressively grinds workpieces, exposing new, sharp gritmaterial, allowing room for chip formation, and effectively increasingthe sharpness or metal removal ability of the wheel.

One prior art publication which discusses the superabrasive wheel andwheel dressing pattern is U.S. Pat. No. 4,653,235, in which a method isdisclosed whereby a watt transducer is utilized to sense grinding wheeldrive motor power consumption over a predetermined time interval at aspark-out or dwell portion of the grinding cycle, and the power levelduring the transition from maximum grinding power to some predeterminedlower power level for re-commencing feed is monitored. If the lowerpower level occurs before the time interval is reached, the wheel isknown to have become too sharp for the intended use, and a dressingcycle is initiated to recondition and "dull" the now too-sharp wheel.The broad concept of monitoring wheel drive motor power consumption iswell known in machines using conventional grinding wheels, because it issimple and inexpensive and it is possible to determine the tangentialforce components from the equation: power equals V_(s) times F_(t),where V_(s) is the grinding wheel speed in meters per second (m/s), andF_(t) is the tangential force (in Newtons). Power monitors have beenvery useful as gap elimination devices and crash detectors, and havebeen used as the bases of adaptive control philosophies. However, powermonitors do not lend themselves to measurement of normal force (F_(n))which is considered imperative for accurate wheel sharpness measurement.Because the coefficient of grinding can vary considerably between dulland sharp wheels, the tangential force is not a reliable indication ofactual normal forces for many working conditions commonly encountered ingrinding operations. Moreover, power monitors, such as watt meters andthe like, have relatively slow response times.

In evaluating wheel sharpness and controlling the grinding processes,other prior art patents have attempted to teach controlling the wheelfeed rate by sensing deflection of the grinding wheel spindle. U.S. Pat.Nos. 3,344,560 (Lillie) and 3,555,741 (Hahn) are two examples of deviceswhich monitor deflection of the grinding wheel spindle to estimategrinding force. The normal force is the principal deflection causingforce in the grinding process, particularly with internal grindingmachines, and some prior art machines have swiveled the wheelhead tore-align the wheel with the workpiece as the grinding wheel spindle isdeflected.

Internal grinding machines are particularly sensitive to deflection ofthe wheel-supporting spindle, which is generally of small diametercompared, for example, with external grinding machines. Controlled forcegrinding has been utilized in order to ensure that a known deflection ofthe spindle could be maintained to control the resulting grindingprocess. Such controlled force grinding has its problems too, however,as eccentric rotation of the workpiece, or irregularities in theworkpiece stock or hardness could cause runout and prevent properround-up of the workpiece. Heavy damping and/or increased wheel rotationvelocities were required to address these problems, which createdproblems of their own, especially in the context of superabrasivegrinding wheels.

An example of additional problems encountered includes post-dressing orpost-trueing inefficiencies. As set forth in U.S. Pat. No. 4,628,643(Gile et al.), following wheel trueing or dressing, the rotationalvelocity of a superabrasive grinding wheel is reduced to maintain asubstantially constant grinding wheel drive power consumption. Thedesired power level is set by the operator, and the rotational velocityis slowly increased in subsequent grinding operations based upon theaverage power consumption and feed rate of the previous grind. Thisprocess slowly increases the grinding output as the superabrasive wheelbecomes sharper (i.e. self-conditions) as described above.

U.S. Pat. No. 4,570,389 (Leitch) addresses post-dressing or post-trueinguse of non-superabrasive wheels. The inefficiencies of this device aresimilar to the Gile device, as following dressing, Leitch teaches thatthe first grind is to be undertaken at a reduced, pre-set feed ratebecause non-superabrasive wheels are sharpened by the dressing process.Perceived sharpness of the wheel is calculated by monitoring changes inhorsepower or normal force between two successive grinds, and over timethe feed rate is increased to the point where the wheel can maintain itssharpness. The increased feed rate tends to self-sharpen thenon-superabrasive wheel enough to offset the dulling effects ofattrition at the optimum feed rate. However, this slow build up of thefeed rate is inefficient. Moreover, this technology is not easilyadapted to superabrasive grinding where in order to maintainsubstantially constant grinding force, the feed rate must be reduced tocompensate for the self-sharpening characteristics of superabrasivewheels.

DISCLOSURE OF THE INVENTION

It is an object of this invention to obviate the above-describedproblems.

It is another object of the present invention to provide an improvedgrinding machine for use with superabrasive grinding wheels which canconstantly monitor the sharpness of the grinding wheel for determiningoptimal feed rates required to maintain a substantially constantgrinding force.

It is yet another object of the present invention to provide a grindingmachine for superabrasive grinding wheels which automatically conditionsthe grinding wheel by monitoring the sharpness of the wheel in order tooptimize the grinding process and output of the machine.

It is also an object of the present invention to provide a more accurateand dependable grinding machine utilizing superabrasive grinding wheelsby accurately monitoring the normal force experienced by the grindingwheel, and by monitoring the sharpness of the wheel in order to optimizethe grinding process.

It is another object of the present invention to provide a grindingmachine which is capable, through monitoring of normal trueing forces,to determine the grinding wheel topography in order to optimize dressingand trueing procedures of that grinding wheel.

In accordance with one aspect of the present invention, there isprovided a system in which a force transducer measures the normal force,F_(n), seen by the grinding wheel, and is used for controlling thegrinding process by methods including:

(a) controlling a constant normal force grind cycle;

(b) evaluating wheel sharpness as a function of normal force, F_(n) andthreshold force F_(th) :

(c) optimizing normal force contact between the grinding wheel andworkpiece at all times;

(d) measuring workpiece run-out while grinding;

(e) optimizing a grind cycle; e.g., through fast, variable ramp-upinfeed;

(f) sensing workpiece contact for gap elimination; and

(g) using threshold force, F_(th) as a basis for adjustment tocompensate for part size discrepancies.

Additionally, applicants have conceived of a control system utilizing anormal force transducer for controlling the wheel conditioning processby methods including:

(a) monitoring contact force between the wheel reshaping tool and thewheel;

(b) monitoring or mapping wheel shape geometry from dressing or trueingforce conditions; and

(c) using wheel sharpness measurement to:

(1) determine dress intervals; and

(2) condition a wheel after trueing by controlling normal force andinfeed rate.

The force measurement system for monitoring the normal force, F_(n),acting on the wheelhead spindle, was chosen to be a very stiff structurewhich would not compromise the existing typical wheelhead mountingarrangement, i.e., stacked plates underneath the wheelhead which areultimately connected to a slide system.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the present invention, it is believed the samewill be better understood from the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic plan view of an internal grinding machine havingthe automatic control structure of the present invention;

FIG. 2 is a partial elevational view taken through a workpiece as shownin FIG. 1;

FIG. 3 is an enlarged perspective view of the grinding wheelhead of FIG.1;

FIG. 3a is a frequency response plot of a load cell wheelhead mount ofthe subject invention and a standard wheelhead mount;

FIG. 3b is a plot of grinding forces determined during a grinding testwith a superabrasive (CBN) wheel;

FIG. 4 is an enlarged plan view of a swivel plate and grinding wheelheadmount contemplated for use in an internal grinding machine such asillustrated in FIG. 1;

FIG. 5 is a graph showing specific metal removal rate (log) versusfinish (log) for a given wheel grade;

FIG. 6 is a graph showing multiple plots of the metal removal parameterlambda versus accumulated volume of metal removed, for plural infeedvelocities during wheel conditioning, for a given wheelgrade;

FIG. 6a is a plot of specific wheel power consumed versus accumulatedvolume of metal removed from a workpiece;

FIG. 7 is a graph plotting normal force versus accumulated volume forplural infeed rates during wheel conditioning;

FIG. 8 is a graph of infeed force per unit time, illustrating acontrolled force system;

FIG. 9 is a graph plotting normal force versus time as actually takenduring a constant grind force cycle;

FIG. 10 is a flow chart illustrating how the target normal force(F_(tar)) is automatically calculated by the subject system;

FIG. 10a is a plot of normal force measurements measured during twowheel dressing procedures, with such force measurements taken across twogrinding wheels clamped together, side by side in both cases;illustrating the sensitivity of the force monitoring capabilities of asystem made in accordance herewith;

FIG. 10b is a graph plotting parts per dress versus the equivalentdiameter of the grinding wheel;

FIG. 11 is the top portion of a flow chart for a preferred grindingsystem:

FIG. 12 is the bottom portion of the flow chart of FIG. 11;

FIG. 12a is an additional portion of the flow chart of FIG. 12;

FIG. 13 is a plot of a CBN grinding pattern, plotting normal force(F_(n)) and Power (K_(w)) versus time (sec.);

FIG. 14 is a schematic diagram of an alternate CBN grinding test setupin accordance with the present invention;

FIG. 15 is a normalized plot of normal force, lambda (×15,000), and timefor a plurality of parts ground with a superabrasive wheel, illustratingthe conditioning process of the wheel following dressing operationsprior to parts 1 and 59; and

FIG. 16 is an enlarged plot of a plurality of CBN grinding forcepatterns, illustrating the enhanced round-up capabilities and processoptimization as the wheel conditions, of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in detail, wherein like numerals indicatethe same elements throughout the views, an exemplary internal grindingmachine 10 is schematically depicted in FIG. 1, wherein machine 10 has aworkhead 11 for supporting and driving a workpiece of revolution 12during the grinding process. The machine 10 has a bed 13 which carriesthe workhead 11, and a grinding wheelhead 14 is mounted to a swivelplate 15, in turn carried by an X-axis slide 16, for moving the grindingwheel 17 radially to the workpiece 12 when performing grindingprocesses. The X-axis slide 16 is carried on a Z-axis slide 18, slidablymounted to the machine bed 13 for providing Z-axis movement, i.e.,axially of the workpiece 12. Suitable X and Z axis servodrives 19,20 areprovided for moving the slides 16 and 18 for relatively feeding wheel17. In a preferred setup, wheel 17 would be fed relative to workpiece12, but this is not critical. A computer numerical control (CNC) 21 isassociated with the machine 10, and serves to provide servo commands inaccordance with programmed instructions, and in accordance with forceparameters detected from a subwheelhead mounting transducer, or loadcell 22 (see FIGS. 3 and 4).

The machine bed 13 also supports a wheel conditioning or reshaping unit23, preferably having a cup-shaped rotary diamond wheel 24, for dressingthe superabrasive grind wheel 17. Slides 16 and 18 similarly provide forrelative feeding movement between wheel 17 and reshaping tool 24, asdesired.

The elevational view of FIG. 2 shows the grinding wheel 17 in a grindingmode, i.e. within the bore of a workpiece 12, wherein X-axis movementwill cause wheel 17 to exert a radial, normal force (F_(n)) on workpiece12. A tangential force (F_(tan)) is also present during grinding. FIG. 3shows the grinding wheelhead 14 and grinding wheel 17, where wheel 17 ismounted to a rotary spindle 25 in the wheelhead 14, and wheelhead 14 iscarried by swivel plate 15, being affixed thereto by an intermediateplate 26 and very stiff load cell 22. As better shown in FIG. 4, loadcell 22 preferably comprises an assembly constructed of fourpiezoelectric devices 27, i.e. solid state members which will yield asignal in response to an applied force. The piezoelectric load cell 22depicted is a commercial assembly available from Kistler InstrumentCorp. of Amherst, N.Y., and is heavily preloaded by means of assemblybolts 28, which establish a preload of several thousand pounds. The loadcell 22 utilized is preferably capable of detecting force variations inthe X and Z directions (see FIG. 1) and also the Y direction (into theplane of the paper as viewed in FIG. 1), i.e., detecting X, Y, and Zmutually-perpendicular force components. The X component, equivalent toF_(n) in FIG. 2, is critical for determining system grind conditions,when considering the factor lambda (λ), which is the volumetric rate ofmetal removal in cubic length units per time unit per unit normal force.

The standard mounting plate (not shown) commonly used in a wheelheadassembly can be discarded, and the load cell 22 replaces it exactlywithout significant loss of machine stiffness. FIG. 3a graphicallyillustrates the substantially identical stiffness achieved by the uniquearrangement of load cell 22 relative to the standard mounting platecommonly used in mounting assemblies. The devices 27 are preferablypreloaded to 0.1MN which ensures transducer linearity and cellstiffness.

The load cell 22 allows normal, tangential, and axial grinding forces tobe accurately and simply obtained. Because cross coupling of signals istypically less than 1% it is not a problem. Therefore, with this system,force measurement fluctuations of less than a fraction of a Newton arediscernible, and accuracy and reliability of the system are enhancedaccordingly.

The problem of signal drift which is inherent in piezoelectric devices,and which is a function within the time frame measured, is computeradjusted by an appropriate compensation algorithm.

Grinding Wheel Spindle

In prior art machines, force measurements depended on compliant elementsthat allowed a discernible displacement proportional to the grindingforce. As discussed above, spindle deflection was commonly monitored todecipher indirectly the forces being exerted by the grinding wheel. Withthe machine of the subject invention, a different approach is taken.

The preferred grinding wheel spindle 25 used in the machine of thesubject invention is a high frequency spindle built with matched pairsof ABEC Class "9" angular contact ball bearings in both the front andrear. Such an arrangement was selected mostly for its stiffnesscharacteristics, and any similar arrangement can be equally substituted.At lower grinding speeds, a 24 krpm spindle was used, having a 14 kwmotor and a static radial stiffness in excess of 90 MN/m. A 45 krpmspindle, with a 9.5 kw motor, was used for higher speeds. The 45 krpmspindle had a static stiffness of 65MN/m. In both cases, the stiffnesswas measured at the spindle nose. Again, specific choice of machinery isnot critical, as long as sufficient stiffness is ensured to allowaccurate force measurements.

Referring to FIGS. 5 and 6, certain parameters have been determined foran exemplary superabrasive grinding wheel grade, as indicated. FIG. 5 isa plot of the logorithms of specific metal removal rate versus finish ofthe workpiece achieved, in RMS (root mean square); while FIG. 6 shows afamily of data curves for various feed rates, in which the value oflambda for any particular feed rate tends to level out as steady-stategrinding (normal force) conditions are reached. From this and similardata, it has been determined that the values of lambda are importantcriteria for determining (a) the increase in wheel sharpness as grindingprogresses, and (b) the conditioning process for optimizing grindingwith an initially "dull" grinding wheel 17 up to and including apreferred lambda or grinding quality range of wheel 17. As indicated,heretofore, a relatively inefficient process of trial and error wasrequired to "condition" a new grinding wheel or a wheel followingdressing or trueing in order to gradually increase the output of themachine up to optimum level.

FIG. 7 shows a plot of normal force versus accumulated volume ingrinding cycles, wherein a family of infeed rates are plotted toillustrate the steady-state condition which is eventually reached with asuperabrasive wheel 17 for each such infeed rate.

FIG. 8 is a graph illustrating a grinding cycle in terms of force versustime, where the grinding wheel approaches rapidly (i.e. is ramped up)and is held at a controlled force by adjusting the feed rate untilinfeed is stopped, then a dwell or spark-out period occurs in which thedecay rate (D.R.) naturally decreases until a threshold force F_(th)occurs in at the end of decay. The threshold force is defined as aninterface force between the grinding wheel and workpiece, at which nogrinding occurs.

FIG. 9 is a graph showing an actual plot of normal grinding force F_(n)versus time. This figure illustrates the substantially instant ramp-upcharacteristics of the subject invention and the substantially constantforce at which grinding is completed. It should be noted that it isimperative that grinding be undertaken at a substantially constant force(e.g. versus at a predetermined feed rate) to achieve a steady statedeflection situation, as will be further described below.

FIG. 10 demonstrates a flow chart of how the target force (F_(tar)) isautomatically calculated. Based upon the monitored change in λ from onepart to the next, the target force (F_(tar)) is calculated for the nextgrind.

As illustrated, a preferred process for determining changes in thenormal force between the grinding wheel and the workpiece during theconditioning of the wheel is based on an inverse proportion compared tochanges in sharpness monitored. Where large changes in sharpness areoccuring, smaller changes in force are necessary to conditioning, andvice-versa. The conditioning force change limit of 10 pounds has beenillustrated only as an example, as other preprogrammed values couldequally be employed.

In addition to the standard criteria describing the composition of avitreous bonded CBN (or other superabrasive) grinding wheel (i.e., gritsize, concentration, bond volume, and porosity) a major factor in adetermination of the wheel's metal removal characteristic is thecondition of the wheel surface.

Immediately after trueing there is little, if any, grain protrusionbecause the bond surface is flush with the grain tips. This causes adull acting wheel with a low metal removal capacity, because there islittle exposed grit and no room for removed metal to escape from betweenthe grinding interface. A dull wheel can be opened, or conditioned, byseveral methods including grit blasting, sticking with a softCarborundum stick, or grinding, usually at reduced rates. The purpose ofeach of these methods is to erode the bond to create sufficient gritprotrusion and chip clearance. None of these methods, however, isefficient or reliably controllable (i.e. consistently repeatable).

When running in a production situation, after a number of grindingcycles, the vitrified CBN wheel will eventually condition itself. Thisself-sharpening will generally continue until a steady-state orintrinsic wheel sharpness is reached. This intrinsic state would be thedesired grinding condition. This condition is difficult to achieve inany repeatable or predictable manner by grit blasting or sticking.Therefore, heretofore, it has been common to approach this state by acontrolled process which usually involves reducing the feed rate after adress-off of a new wheel or reshaping of a used wheel, and graduallyincreasing this rate as the wheel gets sharper. This process wasgenerally trial and error in nature, and was directed to avoiding burnof the workpiece and/or damage to the grinding wheel. Some prior artmachines are capable of automatically performing this conditioning cyclefrom discrete fixed input, however, the present invention provides asystem to do this automatically from feedback of the wheel sharpnessvalue, λ_(w), described below. While wheel sharpness is technicallyλ_(w) divided by wheel speed (V_(s)), because wheel speed will vary onlyminimally between grinds, for the purposes of this discussion, λ_(w)will be understood as equivalent to and used interchangeably with wheelsharpness. It should also be noted that the term lambda may be equallydesignated herein by the symbols Λ, λ or WRP (work removal parameter).

Also, due to low wear rates, a vitrified bonded CBN wheel commonly skipssome number of parts before requiring a re-trueing operation. Whentrueing is completed, however, it is difficult to predict how thetrueing action will affect the wheel's sharpness or cutting capability.The process described below is a method to perform a hands-off,operator-free, automatic wheel conditioning cycle to accommodate thedulling effects of trueing, either after new wheel dress-off or after askip dress.

With successive parts ground, the wheel 17 acts sharper as the bond iseroded. As a result, the force level diminishes for a given feed rateuntil an intrinsic sharpness value is reached. If force is heldsubstantially constant, the sharper wheel removes a greater volume ofmetal per unit time. Two measurements of sharpness were calculated. Onemeasure, the metal removal parameter, λ_(w), is expressed as thevolumetric rate of metal removal per unit force, mm³ /min/N (see FIG.6). Another is specific power expressed as joules/mm³ (see FIG. 6a).Both FIGS. 6 and 6a illustrate the effect of feed rate on the steadystate wheel sharpness (i.e., a steady state wheel sharpness value isapproached for each particular feed rate as the wheel conditions).

The principle employed herein for adaptive grinding with bonded CBNabrasives is centered around the ability to measure the wheel conditioncontinuously via monitoring of the grinding forces, coupled with thefact that, in a stiff machine setup, the value of lambda may becalculated (periodically or continuously) by using wheel/workpiecedisplacement per unit time, diameter of workpiece 12, and the width ofthe cut taken by the wheel 17. Typical output traces from the forcemeasurement system are shown in FIG. 3b. The traces are characteristicand have five distinct phases as indicated on the normal force tracing:(a) approach (non-contact), (b) elastic take-up and force rise(ramp-up), (c) grinding at steady state, (d) spark-out, and (e) thethreshold force level above the non-contact state. The accurate andreliable calculation of wheel sharpness values is a primary achievementof the normal force monitoring of the subject invention; however, thespecific energy, the energy at burn, and optimized spark-out times canalso be determined by this same monitoring structure.

Further, with this system, dressing forces can be measured sensitively(e.g., see FIG. 10a), allowing the topographical condition of the wheelface to be assessed. Thus the control can alert a wheel condition thatexhibits non-uniform hardness, structure, or shape, and eliminate scrapparts due to improper bore profile or taper. In particular, it iscontemplated that the normal force monitored during contact between areshaping tool (e.g. 24 of FIG. 1) and a superabrasive wheel (e.g. 17)taken at various points (laterally across the face) of the wheel can beused to "map" the surface of the wheel. Similarly, following adetermination of the topography of a wheel, appropriate commands couldbe implemented to reshape such wheel so as to minimize reshaping inareas of the wheel where reshaping is not needed. This process wouldlimit the reshaping required, thereby speeding up the process, makingthe dress or trueing operation more accurate, and minimizing unnecessaryreduction of wheel diameter. Proper dressing procedures (i.e. lightcuts) can also help minimize the need for trueing the grinding wheel(i.e. heavier cuts), which may also extend the useful life of a grindingwheel.

Referring to FIG. 3b, it may be noted that steady state maximum forcelevels and steady state decay threshold levels are approached. Based onthe use of vitrified bonded CBN wheels, assumptions for negligible wheelwear and negligible sharpness changes within each cycle were made duringtests. Therefore, in the steady state portion of the grind cycle, thespindle deflection rate is zero and the metal removal rate can be givenby

    Q.sub.w =π[D.sub.w W] [v.sub.f ]

Q_(w) =metal removal rate

D_(w) =workpiece diameter

W=contact width

v_(f) =radial feed rate

Also, if the threshold force level is ascertained empirically forvarious wheel sharpness conditions, then the grinding force decay rateat any point on the spark-out curve can be determined from ##EQU1##λ_(W) =metal removal parameter F_(i) =initial normal spark-out force

F_(th) =threshold normal force

K_(s) =system stiffness

t_(c) =time

By observing this force trend, the condition of the wheel 17 isdeterminable. Hence, the decision to dress can be made based on grindingforces measured during the decay cycle.

Also, as the wheel shape deteriorates, the need to true becomes vital.FIG. 10b shows how the equivalent diameter of the wheel, D_(e), and thedress frequency are related. This can be expressed simply as:

    PPD=A D.sup.B.sub.e

where PPD=parts per dress

A,B=wheel related constants

FIG. 10a shows a plot of normal force measurements taken across twogrinding wheels clamped together, side-by-side, and illustratingdiffering force levels resulting from different radial comp values.

Determining Wheel Sharpness

Using the piezoelectric load cell 22, the machine control is able tomeasure both the normal grinding force, F_(n), and tangential grindingforce, F_(t). From the F_(n) measurement the control can achieve aconstant force grind cycle, as illustrated in the graph of FIG. 9.Because the normal force is held constant, a steady-state deflectionsituation is achieved such that

    v.sub.f =v.sub.s +v.sub.w

where, v_(f) ="X" axis feed rate

v_(s) =rate of wheel radius change

v_(w) =rate of work radius change

In addition, because of the negligible wear rate of a vitrified bondedCBN wheel, v_(s) can be assumed to be 0, leaving

    v.sub.f =v.sub.w

Referring to FIG. 9, the machine control monitors a time interval duringthe constant force portion of the cycle, t₀ to t₁, and also monitors theamount of X-axis feed during this interval, X₀ to X₁. It then calculates##EQU2##

The metal removal parameter lambda, or λ_(w), is described by thefollowing formula ##EQU3## The control will then easily calculate avalue for lambda, since π, D, and W are constants, F_(th) and F_(n) aremeasured, and v_(f) was just calculated. Because λ_(w) is a descriptionof the wheel sharpness, or metal removal capability, the machine controlcan now consult a stored expression or algorithm, derived eitherempirically or analytically, to determine an allowable grind force,F_(n), for the next part to be ground corresponding to the calculatedλ_(w) value. From this new F_(n) value, the corresponding v_(f) valuecan be calculated, since ##EQU4##

The value of λ_(w) can be so calculated to maintain this new F_(n) valueas often as desired, and is contemplated as being calculated for eachrevolution of the workpiece.

Wheel Conditioning

After every new wheel dress, skip dress, or dress on demand, the machinecontrol 21 will automatically begin grinding the next workpiece 12 at apredetermined (or default) test grind normal force, F_(n-test). Duringthis test grind a value for λ_(w) will be determined as described above.This value will then determine the maximum allowable constant F_(n)value, and also the corresponding v_(f) value, for the next workpiece.This procedure will continue until the F_(n) has reached a desiredvalue, usually the value associated with the intrinsic λ_(w) value ofthe grinding wheel 17. In this way, the system automatically optimizesthe grinding process and conditioning of the wheel.

While the terms "trueing" and "dressing" are often used interchangeablyin the prior art, separate definitions are used herein to attain aproperly-conditioned wheel. Specifically, dressing of the wheel is doneprimarily to establish the finish and ability to remove metal (i.e.surface integrity of the wheel), and therefore, "dressing conditions"set in the machine cycle involve light cuts (and compensation therefor)at slow (relative) feed rates of the conditioning tool--for example,diamond nib or wheel--relative to the wheel. In contrast, trueing of thewheel is done primarily to establish the shape of the wheel--eitherwhere new or after use--and therefore, "trueing conditions" set in themachine cycle generally involve heavier cuts (and compensation therefor)at fast (relative) feed rates of the conditioning tool and wheel.

Therefore, a preferred single force grinding method is readily achievedas follows:

Grinding Method--Single Force System Step 1

With reference to FIGS. 5 and 6; on the assumption that trueing makes adull wheel, perform a sharpness test on a given wheel type or grade--forexample, an XBN 100T 100VHA--to determine:

a. λ of a dull wheel, i.e., λ exp (lambda expected)

b. F_(th) of a true wheel, i.e., F_(th) max

c. F_(nss) (normal force at steady state) at Z_(w) that produces therequired RMS finish, i.e., F_(n). ##EQU5##

Step 2

Select:

a. Approach factor for Ramp Feed Rate, i.e., N₁

b. ##EQU6## value to describe F_(th), i.e., D.R. c. C.F. (ControlledForce) Rate Factor to maintain grind force, i.e., N₂

d. λ upper and lower limits for skip dress, i.e., λ_(u), λ_(L) (λ_(L)could be lower than λ_(min)).

Step 3

Determine grind parameters following multiple pass dress: ##EQU7## b.Ramp Rate=N₁ ×V_(f), i.e. V_(AD)

Step 4

Monitor actual wheel sharpness parameters during grind to determinerates for next cycle:

a. measure ##EQU8## cycle during constant force portion of cycle b.measure F_(th) at predetermined decay rate value and adjust finish grindposition accordingly

c. calculate actual λ, where ##EQU9## d. compare λ act to λ_(U) andλ_(L) to determine if skip dress is required.

Step 5

Determine grind parameters for subsequent revolutions of the workpiece##EQU10## i.e., V_(f) b. Ramp Rate=V_(f) ×N₁, i.e. V_(AD)

c. Spark-out time determined by D.R. setting

Step 6

Continue steps 4 and 5 within the present grind cycle and subsequentgrind cycles until a multiple pass dress is performed.

The method and apparatus disclosed herein are the outgrowth of agrinding test setup designed to evaluate superabrasive wheels--inparticular CBN wheels.

CBN GRINDING TEST SETUP AND DATA ACQUISITION TECHNIQUE

Computerized data acquisition is an extremely powerful modernengineering tool. Analog or digital parameters can be electronicallyinterfaced with personal computers for analysis. The desired triggeringand timing can be easily programmed, and formulas can be applied to theinputs before they are stored. Once the data are collected, othervariables can be generated as a function of the measured items. Valuablegraphs can also be generated to show descriptively the relationshipsbetween the parameters. Because of the high resolution of the data,these graphs can be expanded to examine closely a particular area ingreater detail. It is also contemplated that computer artificialintelligence could advantageously be adapted to further enhance thisinvention. For example, the computer could determine initial feed rates,critical normal force and lambda values, and, possibly, determine skipdress cycles based upon its "learnings" over a plurality of grindingcycles and "experience" with particular types and grades of grindingwheels.

The following procedure describes how a data acquisition system has beenapplied to test the internal grinding machine of the present invention.This data acquisition process is used to evaluate wheels, grindingfluids, dressers, and most importantly, provide the basis for theadaptive control philosophy.

Test Data Acquisition Equipment

With reference to FIG. 14, a schematic diagram, the principal elementsof the test data acquisition system are,

a) a set of piezoelectric force rings (load cell 22), mounted under thegrinding wheelhead 14 and spindle 25 for measuring grinding forces,

b) a power monitor (watt transducer 29) for determining wheelhead powerconsumption, and for cross-referencing tangential forces experienced bythe grinding wheel F_(t),

c) a linear variable differential transformer (LVDT) 30 or similarlinear feedback device for monitoring the X-axis position of thegrinding wheel,

d) a proximity switch 31 mounted at the rear of the workhead 11 fortriggering the data collection activity,

e) a 32 bit microcomputer, PC32, with hard disk and floppy disk drivesfor collecting and storing the data, and

f) signal conditioner boards 33, amplifiers 34 and filters 35.

Signal Processing

Four parameters were measured

1) LVDT - X slide position

2) HP - wheelhead power

3) F_(tan) or F_(t) - tangential grinding force

4) F_(n) - normal grinding force

The normal and tangential forces are measured during each revolution ofworkpiece 12 by the piezoelectric devices 27 of the load cell 22. Thepiezoelectric circuitry (not shown) has provisions to null whencommanded, and it is preferred that the devices 27 be nulled prior toeach grinding cycle.

The two measured force signals are then sent to a low pass filter 35 toeliminate variations due to wheelhead rotational speed. Both of thesesignals, along with the horsepower signal from the power monitor 29 andthe displacement signal from the LVDT 30, are fed into the signalconditioner 33. Signal conditioner 33 minimizes the noise generated bythe servo drives 19 and 20 and also adds a calibration point. The foursignals are fed to both the personal computer, PC32, and a strip chartrecorder 36.

The interface to the PC32 is via the data acquisition board and adapter37. Two additional inputs into the PC32 are a start acquisition signaland the workhead proximity switch 31. The strip chart recorder 36 wasused during the test merely for comparison. The recorder 36 has startand stop inputs available.

Procedure to Acquire Data

Initially, data were collected every 0.025 seconds (i.e. five times perrevolution of workpiece 12 being rotated at 480 rpm). This resulted intoo much data being collected. The solution to this problem was toinstall the a workhead proximity switch or trigger 31 as a triggersensor to assure synchronization of acquisition with work speed. It wasnot important whether the maximum force, minimum force or some interimforce of each revolution was being recorded, because all grind cycleswere purposely set up so that steady state forces were achieved and thepart was properly rounded up.

Just before the (CNC) machine controller 38 started the feed rate grind,a command was sent from the controller 38 to start the strip chartrecorder 36. Half a second later the command was given to null thepiezoelectric circuit associated with the load cell 22. This eliminatedprevious signal drift.

Simultaneously, the data acquisition software was commanded to start thedata collection. When the signal was received, it collected data eachtime the workhead trigger was sensed (preferably each revolution ofworkpiece 12). One set of four values was then stored in memory. Theseinputs were analog values. Negative ten volts to positive ten volts wererepresented by a value of 0 to 4095. Each time the proximity switch 31was sensed, another set of data was stored. This continued until thecontroller 38 removed the start acquisition command after completing thegrind. Since the work speed was 480 RPM, data collection occurred eighttimes per second.

Once the grind cycle was completed, the acquisition software convertedthe raw data to actual values of mm, watts and newtons. The conversionshad been previously specified by a programmable formula for each input.Then the values were automatically put into a spreadsheet at previouslydefined cell locations. At this point, the acquisition software hadcompleted its task.

Now the spreadsheet took over using its macro capabilities. Normalforce, tangential force, and the threshold force were sequentiallydisplayed on the computer, PC32, as graphs to assure the integrity ofthe data. FIG. 3b shows a sample of F_(n) and F_(t) data collected forone grinding cycle. Based on all the data in the spreadsheet, manycalculations were performed to determine normal force, lambda and timerelationships, as set forth above. Maximum values of the steady stateF_(n), F_(t) and Power (HP) were determined and used for thecalculations.

A report file of each grind was imported into a summary spreadsheet.Table 1 contains sample data that were saved on the hard disk in areport file from which summary data sheets were generated.

                  TABLE 1                                                         ______________________________________                                        Report Summary for test Wheel I5 at 1.905 mm/min (.075 IPM)                         spec pwr                    lambda                                      grind joules/  F.sub.n max                                                                           F.sub.t max                                                                         F th mm.sup.3 /                                                                          finish acc vol                        no    mm.sup.3 N       N     N    min/N Ra   cm.sup.3                         ______________________________________                                        I5r1  123.5    773     286   98   10.8  .31  26.60                            I5r2  98.3     645     233   89   13.1  .33  27.81                            I5r3  77.0     573     199   85   14.9  .33  29.02                            I5r4  71.5     556     188   85   15.4  .33  30.25                            I5r5  71.0     536     182   80   16.9  .34  31.46                            I5r6  67.5     527     176   80   16.3  .34  32.68                            I5r7  67.5     522     175   80   16.5  .34  33.89                            I5r8  66.4     523     173   80   16.5  .34  35.10                            I5r9  65.4     506     174   80   17.1  .35  36.31                             I5r10                                                                              65.4     511     171        16.9  .35  37.53                             I5r11                                                                              65.4     512     171   80   16.9  .34  38.74                             I5r12                                                                              66.0     516     171   80   16.8  .35  39.97                             I5r13                                                                              65.7     510     170   80   16.9  .35  41.18                             I5r14                                                                              65.6     507     171   80   17.1  .36  42.39                             I5r15                                                                              66.1     506     171   80   17.1  .36  43.61                            ______________________________________                                    

The normal force may also be used to trigger the end of a gap eliminatorrate, i.e., where a wheelslide is advanced to the workpiece at a maximumrate, to avoid "cutting air" at a grinding feed rate. When the wheelcontacts the workpiece, the gap rate is dropped out and the machine goesinto a coarse grinding rate.

The flow chart of FIGS. 11, 12 and 12a show a preferred productiongrinding cycle controlled in response to monitored normal force, andincluding a gap eliminator rate as contemplated above. In particular,FIG. 11 shows the contemplated sequence of implementing the grindingmachine and process of the subject's invention wherein a workpiece 12and a grinding wheel 17 are loaded into a grinding machine such as shownin FIG. 1. If this is the first grind after trueing, dressing, orloading of a new grinding wheel, the initial values for lambda and thethreshold force (F_(th)) are used. If this is not the first grind, themeasured values for these parameters are used. A target grind force(F_(t)) and an initial ramp rate (IRR) are initially set based uponexperience factors and/or historical data concerning a particular typeand grade of grinding wheel, and the grinding wheel and workpiece aremoved relative one another at the gap eliminator rate mentioned above.This gap eliminator rate is implemented until the normal force monitoredby loadcell 22 indicates when a small contact force between theworkpiece and wheel has been sensed.

As set forth in FIG. 11, during the grinding process, the maximum normalforce (F_(n-max)) and the minimum normal force (F_(n-min)) is monitoredand recorded for each revolution of workpiece 12. The feed rate can bechanged after each revolution as necessary in inverse proportion to therelationship of the maximum normal force to the target grind force(F_(t) /F_(n-max)). In this way, the feed rate is automatically adjustedto quickly ramp up to the target force, thereby optimizing the grindingprocess by insuring that grinding is undertaken at the optimum normalforce according to the condition of the grinding wheel during eachrevolution of the workpiece.

The minimum normal force monitored during any particular revolution isalso utilized to insure that a minimum force level (F_(clean)) has beenmet (i.e. to insure that the grinding wheel has made contact with theentire inside diameter surface of the workpiece). Once the minimumnormal force exceeds the F_(clean) force level, measurements can becommenced to calculate the values of lambda in accordance with thediscussion above. This data is monitored for each revolution of theworkpiece until the grinding wheel has reached the finished sizeposition as indicated by the x-axis slide position. As will be discussedbelow, in order to insure that the cut part has been rounded up prior toreaching its finished size, the subroutine shown in FIG. 12a ispreferably implemented.

When the x-axis slide has reached the finished position, the feed isstopped, and spark-out to a maintained normal force condition (F_(th))ensues. The normal force measurements can be ended when the feed hasbeen stopped, or can be continued to monitor spark-out force to thethreshold normal force. Thereafter, a new value for lambda is calculatedbased upon the information collected during the grinding cycle, and thisnew value of lambda and the measured value of the threshold force(F_(th)) will be used for the next grinding cycle.

As set forth in FIG. 12, following a grind cycle, a skip dress counteris incremented, and the machine determines whether the grinding wheelmust be dressed. A need for dressing the grinding wheel may be indicatedafter a pre-determined number of parts have been ground, or when thecalculated value of lambda for the grinding wheel exceeds apredetermined critical value. A critical value for lambda can bepredetermined based upon experience or data concerning values for lambdaabove which the wheel may break down in use or lose shape or size due toexcessive bond erosion.

FIG. 12 also illustrates the additional advantage of the unique normalforce monitoring capabilities of the present invention, as normal forceof contact between a reshaping tool and the grinding wheel can bemonitored to insure that these procedures are properly completed.Moreover, the grinding wheel topography or geometry can be determined or"mapped" based upon the normal force values indicated during thedressing and/or trueing procedures. It is contemplated that such mappingcan be accomplished either transversely across the grinding surface of awheel, or circumferentially about the periphery of the wheel to insureroundness. With appropriate machine response capabilities, by varyingthe feed movement in accordance with the values of the normal forcesmonitored, the reshaping process can be optimized by minimizing thenormal forces in areas of the grinding wheel where reshaping is notneeded.

Out-of-Roundness Detection

The load cell 22 may also be used for sampling variations in normalforce around the grind periphery, and thereby determine out-of-roundnessof the workpiece 12. In tests, as many as 30 intermittent force valueshave been sampled on the periphery of workpiece 12. Again, in machineswith appropriate response capabilities, the infeed can be varied inproportion to the sampled force values to round-up the workpiece 12. Apreferred procedure for determining and compensating forout-of-roundness is shown in FIG. 12a.

As mentioned above, before the grinding wheel has reached the finishedsize position, it is important to insure that the workpiece has beenrounded up. Therefore, it is preferred that before the finished sizeposition is reached, the maximum normal force (F_(n-max)) for eachrevolution of the workpiece compared with a normal force limit valuewhich has been preset into the machine (e.g., a normal force value 20lbs. higher than the constant force or target grind force, F_(tar),established for that grind cycle). If the maximum normal force exceedsthis safety limit, the feed is stopped, as there may be some problemwith the machine. In actual use, the maximum normal force will neverexceed this normal force limit unless there is a problem which must beaddressed by the operator.

As long as the maximum normal force is less than the normal force limit,the feed rate is optimized during each revolution of the workpiece byadjusting the feed rate in inverse proportion to how close the maximumnormal force (F_(n-max)) is to the target normal force (F_(tar)). Thesystem keeps track of the force during each grind cycle by accumulatingthe maximum and minimum normal forces monitored during each revolutionduring the grind cycle, and averaging these forces for use indetermining the value of lambda. The roundness of a part being ground isdetermined by dividing the difference between the maximum and minimumnormal forces experienced in a particular revolution by the stiffness ofthe system (K), which is a known constant. The amount of run-out iscompared with a pre-determined run-out tolerance limit set in themachine, and if the run-out in the part is less than the tolerance, aroundness flag is set in the software so that it is known that theroundness requirement is met.

It should be noted that the unique adjustment of feed rate provided bythis system can be used to help insure that the part will be ground toroundness prior to running out of stock. It is contemplated that thenormal force monitored can be utilized to compare the amount of run-outindicated in the part to the amount of stock left in the part, so thatthe machine can slow down the feed rate to insure that the part isrounded within the grinding and spark-out portions of the process. Thisadvantage enables more efficient production of parts and reduction ofrejects.

FIG. 16 graphically illustrates tracings of controlled force grindingsundertaken in accordance with the subject invention, and the automaticconditioning process provided by this grinding apparatus. In particular,the first tracing of FIG. 16 shows the first grind after a dressingprocedure, and illustrates graphically the rounding-up process describedabove. In particular, the darker initial areas of the tracing representthe normal forces monitored, wherein the maximum normal forces aresubstantially higher than the minimum normal forces. The tracings ofFIG. 16 correspond to the normal force tracing shown and described withregard to FIG. 3b, and illustrate how various parts having differentrun-out are rounded up relatively quickly by the present controlledgrinding process, and well prior to spark-out. The tracings of FIG. 16further illustrate the self-conditioning process of the presentinvention wherein the normal forces are automatically optimized asdiscussed.

While the invention has been shown and described in connection with apreferred embodiment, it is not intended that the invention be solimited; rather, the invention extends to all designs and modificationsas come within the scope of the appended claims.

What is claimed is:
 1. A grinding machine having a machine base, awheelhead supported by said machine base and carrying a rotatablesuperabrasive grinding wheel having a peripheral grinding surface, awheel shaping unit mounted on said base including feed means forrelatively moving said grinding wheel and wheel shaping unit withrespect to one another at a selected feed rate, and an apparatus fordetermining wheel topography during wheel reshaping operations, saidapparatus comprising means for measuring the presence and magnitude of anormal force vector occurring between said wheel and said wheel shapingunit, said measuring means including a force transducer mounted adjacentsaid wheelhead, wherein said determined wheel topography is based uponnormal force vectors measured at a plurality of points about saidperipheral grinding surface, and means for automatically adjusting saidfeed rate in accordance with the value of said measured normal forcevector to optimize the wheel shaping process and to minimize said normalforce vector where reshaping is not needed.
 2. The grinding machine ofclaim 1, wherein said means for adjusting said feed rate furthercomprises machine response capabilities for automatically varying theamount of relative movement and frequency of such movement between saidwheel reshaping unit and said grinding wheel in response to one or moremeasured magnitudes of normal force vectors such that said wheel shapingunit can effectively reshape said grinding wheel as desired.
 3. Thegrinding machine of claim 1, wherein said force transducer comprises apiezoelectric element capable of detecting force variations in threemutually perpendicular directions.
 4. A method for accurately reshapinga superabrasive grinding wheel in a grinding machine, said methodincluding the steps of:(a) rotating a superabrasive grinding wheel on awheelhead; (b) providing a wheel reshaping tool in proximity to saidgrinding wheel; and (c) effectuating relative feed movement between saidwheelhead and said reshaping tool to cause contact of said wheel withsaid tool; (d) providing a normal force transducer for detecting normalcontact force between said wheel and reshaping tool; (e) monitoring theresulting normal force between said wheel and reshaping tool while saidrelative feed movement is occurring to determine the topography of saidgrinding wheel; and (f) automatically varying said feed movement inaccordance with the values of said normal contact force to optimize thereshaping process and to minimize the normal forces in areas of saidwheel where reshaping is not required.
 5. The method of claim 4, furthercomprising the step of effectuating the relative feed movement betweenthe wheelhead and the reshaping tool at periodic intervals, and varyingthe frequency of said periodic intervals in accordance with the valuesof said normal contact force to minimize reshaping in areas of saidwheel where reshaping is not needed.
 6. The method of claim 4, whereinstep (d) includes locating said normal force transducer within awheelhead mounting member.
 7. The method of claim 6, wherein step (d)further includes using a piezoelectric load cell as said normal forcetransducer.
 8. The method of claim 4, further comprising the step ofdetermining the topography of said wheel based upon the normal forcesmeasured at a plurality of points on said wheel, said topography beingdetermined prior to the step of varying the feed rate to optimize thereshaping process.
 9. The grinding apparatus of claim 1, wherein saidapparatus for determining wheel topography can effectively map thegrinding surface of said grinding wheel transversely across saidgrinding surface and circumferentially about the periphery of saidwheel, as desired, and wherein said means for adjusting said feed ratecan vary said feed rate according to the values of said normal forcevector to optimize the reshaping process of said wheel accordingly. 10.The grinding machine of claim 1, further comprising means forcontrolling the reshaping process of said grinding wheel, saidcontrolling means connected to said means for measuring said normalforce vector, whereby upon measurement of a minimum contact forcebetween said wheel shaping unit and said wheel, said controlling meansinitiates determination of said wheel topography.
 11. The method ofclaim 4, wherein said step of monitoring the resulting normal force isinitiated once said normal force transducer detects a minimum contactforce between said wheel reshaping tool and said grinding wheel.
 12. Themethod of claim 4, wherein said step of monitoring the resulting normalforce comprises determining the topography transversely across thegrinding surface of said grinding wheel.
 13. The method of claim 4,wherein said step of monitoring the resulting normal force comprisesdetermining the topography circumferentially about the periphery of saidgrinding wheel.